The invention herein described pertains generally to electromagnetic energy detection, and more specifically to apparatus for sensing and displaying the direction of arrival of characterized electromagnetic energy. The invention has particular applicability to compact portable apparatus for reliably identifying a target illuminated by a pulsed laser beam.
Since the development of practical laser apparatus, such apparatus has taken a variety of forms and is used to perform a rapidly increasing number of functions. Continuing developments have led to the use of lasers in increasingly sophisticated, complex and compact apparatus. Laser energy is uniquely adapted to perform many heretofore impractical functions because of its coherent, extremely stable, single frequency characteristics, thus making possible the generation and transmission of very well defined and characterized beams of energy.
One of the practical present uses of a laser beam is to mark or identify a remote object or target. Since a very narrow laser beam can be produced and transmitted, marking or identification can be accomplished with very great resolution. Laser energy reflected from the target can then be detected by apparatus remote from both the transmitter and target, and used to reliably identify the target. Such a system can be used by pilots or other users in separate aircraft to provide means whereby those in one aircraft can accurately identify a ground target to persons in other aircraft.
One of the problems encountered in such applications is that objects between the transmitter and intended ground target, such as clouds and/or other airborne particles, may cause a partial reflection or dispersion of the laser beam. Laser energy arriving from such particles or objects may cause confusion and result in erroneous target identification. Thus, for maximum reliability it is necessary to provide means for discriminating between energy received from valid and invalid targets. In aircraft applications in which space and weight capacity is limited, and in which interference with and additional demands placed on an operator must be kept to a minimum, it is also necessary that the detection and display apparatus be compact, simple to operate, and cause minimum interference with other activities.
The detection and display apparatus of the applicant's invention fulfills the noted requirements by means of unique signal discrimination circuitry which provides for reliable discrimination between signals resulting from valid and invalid targets, and by utilizing signals corresponding to valid targets to produce an easily understandable target location display. The apparatus is self-contained and compact so that it may be conveniently carried on a user's person. The display may be in the form of a virtual image before the user's eye so as to provide minimum interference with his other visual activities.